I'm just finishing up a restoration of a late 1956 Gibson GA40 Les Paul amp, a real basket-case, so I've had to get deep into the details. As with almost all Gibson amps, there are variations from the schematic. I've found two schematics for this amp online, and the actual amp is mostly like one, but contains elements of both. I'm curious as to which schematic came first since they are not dated.
This is basically the amp I've been working on in terms of the input resistor arrangement, the Channel 1 biasing, the volume pot direction, the mixing resistors, and the tone pot wiring between the 12AX7 plates.
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But it contains elements of this more commonly available schematic:
![Click image for larger version
Name: GA-40LesPaul.JPG
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The biasing of the Channel 2 Tremolo input stage including the Tremolo circuit correspond to this second (or earlier?) schematic.
The only other photos of this inside of this amp I can find online correspond to the one I've been working on.
My guess is that the second schematic above is the earliest version of this amp and that most of the changes shown on the first schematic had been implemented by 1956.
Edit: A picture I found online of a 1961 GA40 suggests that I may have the revision sequence backwards.
This is basically the amp I've been working on in terms of the input resistor arrangement, the Channel 1 biasing, the volume pot direction, the mixing resistors, and the tone pot wiring between the 12AX7 plates.
But it contains elements of this more commonly available schematic:
The biasing of the Channel 2 Tremolo input stage including the Tremolo circuit correspond to this second (or earlier?) schematic.
The only other photos of this inside of this amp I can find online correspond to the one I've been working on.
My guess is that the second schematic above is the earliest version of this amp and that most of the changes shown on the first schematic had been implemented by 1956.
Edit: A picture I found online of a 1961 GA40 suggests that I may have the revision sequence backwards.
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